Aircraft: MEDIUM TRANSPORT; HIGH WING; 2 TURBOPROP ENG
Phase of flight: Initial Approach
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT

BHM approach cleared us direct to SPATT 'for now' and to expect the ILS 24 approach. This clearance set us on an approximate course of 100 degrees to SPATT and the 'for now' part of the clearance placed a biased expectation in my mind that at some point ATC would give us a left turn vector to parallel the localizer course outbound before turning us inbound. This was in keeping with how this approach has been conducted in the past. At this point we were descending to 3000 ft. As we got closer to SPATT; I commented to the FO that ATC needed to turn us out soon as the turn inbound from SPATT was way too tight. This indicates the mindset of my expectation. About 4 NM from SPATT ATC assigned us a right turn heading 180 to join the localizer; maintain 2600 feet until established. This was unexpected by me and I realized that it was a close-in turn onto the approach. I immediately pulled the power back to slow down but we soon were crossing the localizer. ATC; aware of this; gave us a heading of 270 to rejoin. We rejoined the localizer inside of HUKEV. Somewhere in the automation selection; I lost the vertical command bar in the turn to try to intercept the localizer and I lost vertical situational awareness to the extent that a 'TERRAIN AHEAD' caution was initiated due; I think; to the rate of descent. I descended to 2000 feet where the minimum altitude was 2200 feet at LOWGA. I initiated a climb to an appropriate altitude on the approach as the TAWS (Terrain Avoidance Warning System) display displayed only green terrain contours as soon as the climb was started. The approach was stabilized by the time we arrived at LOWGA and the rest of the approach and landing were normal. Crossing the localizer and rushing to get established and configured. TAWS alert. Because of a biased expectation about how the flight would be set up for the approach; I found myself going from a place of comfort with the approach to suddenly being severely behind the 'eight ball'.As we were nearing SPATT; the thought occurred to me to query ATC as to what their plan was for vectoring me onto the approach. I should have acted upon that thought. As soon as I received the heading I did not expect and realized that it was a tight turn-on creating a rushing situation; I should have abandoned the approach and requested vectors back around instead of trying to salvage the approach. This would have preempted the problems that occurred with the rushing that led to automation confusion and altitude deviation. We were vectored for an ILS 24 approach into BHM. I identified the localizer and announced it was identified on both sides. We were on a heading approximately 110 degrees navigating direct to SPATT intersection (12.2nm final for runway 24) using LNAV mode and cleared down to 3;000 still in the descent. Since it looked like a very sharp turn necessary to get established on the localizer; the Captain and I both thought the controller would turn us left to make a downwind leg first. Instead the controller turned us sharply right to 180 degrees and cleared us to join the localizer and descend and maintain 2;600. The Captain had the speed up at 220 knots and made the turn and descent. This was a 60-degree intercept onto the localizer. The captain was still in RNAV mode and must have pushed the NAV button on the [FGS] because the lateral mode of the FD dumped. We were in basic mode for roll. I said; 'We are in Basic Mode on the lateral mode.' The captain did and said nothing. I called 'Localizer Alive...it will not capture...LOC is NOT armed.' The Captain then started pushing buttons on the [FGS] and LNAV engaged and turned the airplane to parallel the localizer. I said; 'We need to turn right to join the localizer.' ATC saw we went through the localizer and issued a 270 degree heading to join and maintain 2;600 until established and cleared for the approach. I set the heading bug. The Captain pushed some more buttons on the [FGS] and the FD lateral mode went again to basic mode. Then he disengaged the AP to make the turn and failed to apply back-pressure to maintain altitude. I was so focused on him keeping us on course; that I failed to notice the altitude loss. The TAWS (Terrain Avoidance Warning System) sounded once; 'Terrain Ahead'. I immediately said; 'We need to climb NOW.' We were at 2;000 when the climb began and we leveled off at 2;600 and intercepted the GS and made a normal approach and landing. We de-briefed extensively after the flight.We didn't expect the controller to slam dunk us on the sharp turn to final. The Captain had the speed up as a result. We were in LNAV mode navigating direct to SPATT. V/L (Vertical/Lateral) was not active on the Captain's EFIS. The approach checklist was not called until after intercepting the localizer and glideslope. We had mode confusion from the Captain freaking out and pushing buttons on the [FGS] trying to figure why we were not joining the localizer.Over-reliance on the automation: The Captain did not disconnect and fly raw-data until the situation was out of hand. Failure to properly monitor the situation: I simply could not keep up with the rapid button pushing the Captain was doing. I had no idea what he was doing; what mode he was in; or what he was trying to do. It was so many errors occurring at the same time that I became task-saturated. Standardize the ILS profile to include guidance for when to go to V/L mode and use Heading Mode instead of using LNAV to navigate. This should be XX miles/minutes from the initial fix/final fix/localizer cross-track distance. Turboprop flight crew reports an unexpected turn to intercept to the localizer at BHM resulting in the Captain overshooting the LOC and descending 600 feet below the assigned altitude of 2;600 feet. A Terrain Avoidance Warning System (TAWS) 'Terrain Ahead' warning is annunciated and the crew climbs back to 2;600 feet.